Optical receivers are used in a large number of light sensing applications. The receiver typically includes a photodetector that is illuminated by a light signal and generates a current that is related to the intensity of the light. This current is converted to a voltage by an interface circuit to provide an output signal that is utilized by an apparatus connected to the optical receiver.
In some light sensing applications, an output signal that is proportional to the logarithm of the light intensity is particularly useful. For example, the apparatus that is processing the output signal may need to compute the product or ratio of two light signals. This computation can be performed on the logarithmic signals using simple addition or subtraction circuits.
In other light sensing applications, the amplitude of the light signal can vary by a factor of more than 100,000. Providing an output signal that is proportional to the intensity of light in these situations is not practical, since the circuitry that processes the light signals has a fixed maximum potential and sensitivity. If the circuitry is set to provide the maximum output at the highest intensity, then the sensitivity of the circuitry at the lowest light levels is insufficient. In this regard, it should be noted that many circuit elements have intrinsic noise that overwhelms very small signals. If, on the other hand, the circuits are designed to provide adequate sensitivity at the low light levels, the circuits will saturate at the high light levels and differences in intensity levels at high light intensities cannot be measured. One solution to this problem is to provide an output signal that is related to the logarithm of the light intensity. Such a signal provides the required sensitivity at all light levels.
A transimpedance amplifier (TIA) is often used to convert the current signal from devices such as photodiodes to a voltage signal that is then utilized by other circuits. TIA designs that convert the input current signal to a voltage that is proportional to the logarithm of the input current could, in principle, be constructed by replacing the feedback resistor in the conventional TIA with a diode. However, such circuits have stability problems. To overcome these stability problems in applications requiring a very high dynamic range, the amplifier must have a very high frequency capability that can only be achieved at power levels that make such TIAs impractical for many applications.